Reason for acting in new movie, capturing actor's essence (6)
I believe the answer is:
motive
'reason for acting' is the definition.
(a motive is a reason for doing something)
'new movie capturing actor's essence' is the wordplay.
'new' indicates anagramming the letters.
'capturing' is an insertion indicator.
'essence' says to take the centre (essence can mean the core of something).
The middle of 'actor' is 't'.
'movie' is an anagram of 'moive'.
'moive' enclosing 't' is 'MOTIVE'.
'in' is the link.
(Other definitions for motive that I've seen before include "What induces one to take a certain action" , "Purpose in doing something" , "Rationale, purpose" , "Reason for committing a crime" , "Underlying intention" .)